1. Field of the Invention
The present invention relates to the field of backlight driving of liquid crystal display, and in particular to an LED (Light-Emitting Diode) backlight drive circuit that helps reduce the size of an LED constant-current driving chip.
2. The Related Arts
Nowadays, science and technology are in rapid progress and various innovated information devices are available in the market to suit the needs of general people. In early days, displaying devices are mostly CRD (Cathode Ray Tube) displays, which have a bulky size and consumes a large amount of electrical power and generate radiation so that long term use of the displays is hazardous to body health for users. Thus, the displays that are currently available in the market are gradually switched from liquid crystal displays (LCDs) that substitute the CRT displays used in early days.
LCDs have a variety of advantages, such as thin device body, low power consumption, and being free of radiation, and are thus widely used. Most of the LCDs that are currently available in the market are backlighting LCDs, which comprise a liquid crystal panel and a backlight module. The operation principle of the liquid crystal panel is that liquid crystal molecules are interposed between two parallel glass substrates and a drive voltage is applied to the two glass substrates to control the liquid crystal molecules to change direction in order to refract out light emitting from the backlight module for generating images. Since the liquid crystal panel itself does not emit light, light must be provided by the backlight module in order to normally display images. Thus, the backlight module is one of the key components of an LCD. The backlight module can be classified in two types, namely side-edge backlight module and direct backlight module, according to the location where light gets incident. The direct backlight module comprises a light source, such as a cold cathode fluorescent lamp (CCFL) or a light-emitting diode (LED), which is arranged at the back side of the liquid crystal panel to form a planar light source that directly provides lighting to the liquid crystal panel. The side-edge backlight module comprises a backlighting source of an LED light bar arranged at an edge of a backplane that is located rearward of one side of the liquid crystal panel. The LED light bar emits light that enters a light guide plate (LGP) through a light incident face of the light guide plate and is projected out through a light emergence face of the light guide plate, after being reflected and diffused, to thereby pass through an optic film assembly to form a planar light source for the liquid crystal display panel.
The backlight module comprises a backlight drive circuit that drives an LED light string to emit light so as to provide the backlighting. A conventional backlight drive circuit comprises an LED constant-current driving chip. The LED constant-current driving chip comprises an output over-voltage protection function, whereby when open-circuit of the LED string happens in the circuit, the LED constant-current driving chip detects zero voltage at a negative terminal of the LED light string and the LED constant-current driving chip determines that this is caused by insufficiency of output voltage and will increase the duty ratio of drive signal supplied to MOS transistors. The greater the duty ratio is, the greater the output voltage of the backlight drive circuit will be. Consequently, the output voltage rises. Since the LED light string is open-circuited, the LED constant-current driving chip detects no voltage at the negative terminal of the LED light string and will control the output voltage to unlimitedly rise. If no output over-voltage protection function is provided, the output voltage will get up continuously and will then exceed the durable voltage of the components in a back-side circuit. The components will be break through and even a fire will be caused, resulting in risk of operation safety.
As shown in FIG. 1, an LED constant-current driving chip 10 comprises an output over-voltage protection module 12 that is integrated inside the LED constant-current driving chip 10. The LED constant-current driving chip 10 contains therein a comparator 14. The comparator 14 has a positive input connected to an over-voltage protection (OVP) pin of the LED constant-current driving chip 10 and a negative input connected to constant reference voltage V that is generated by the LED constant-current driving chip 10 when receiving application of electrical power thereto. When an abnormal rise of output voltage occurs, a detection voltage obtained through voltage division realized with resistors R10 and R20 is supplied to the OVP pin to compare with the constant reference voltage V that is supplied to the negative input of the comparator 14 within the LED constant-current driving chip 10. When the detected voltage is greater than the constant reference voltage V, the comparator 14 supplies an output of high level to activate the output over-voltage protection module 12 of the LED constant-current driving chip 10 and cut off a drive signal of a MOS transistor Q10, so that the output voltage of the backlight drive circuit is prevented from further rising and the components of circuits are protected from being broken by high voltage. However, the process shows the following drawbacks:                (1) OVP pin of the LED constant-current driving chip 10 is used and this is adverse to compaction and size reduction of LED constant-current driving chip 10.        (2) When shorting of the division resistor R10 or open-circuiting of the division voltage R20 occurs, the high output voltage will be directly applied to the LED constant-current driving chip 10 and thus damage the LED constant-current driving chip 10.        